Earthquake and like vibratory phenomena are capable of breaking fluid pipe lines for petroleum and gases or grounding electrical transmission lines, thus giving rise to such secondary hazards as explosions, fires and the like.
Accordingly, a switching means for automatically closing or opening a related circuit, in response to vibrations above a predetermined magnitude, in which the circuit can be manually restored to the initial state for restoring the related circuit, has been desired. Also, such means should be desirably compact in construction and have a long service life.
Heretofore, various types of such means have been developed. However, those utilizing electricity or magnetism encounter difficulties in use in view of power stoppage which may be caused at the time of an earthquake, leading to operational failure at the time of the emergency and they also encounter difficulties in daily inspection and maintenance. Therefore, their use is limited to special cases. There have also been developed various mechanical switching means to the same end, and they include one, which utilizes a ball placed on a horizontal surface and adapted to roll and/or fall at the time of an earthquake, thereby closing a related circuit, and another, which utilizes a bar vertically placed on a horizontal surface and capable of falling at the time of an earthquake to close a related circuit.
Although these mechanical switching means have been generally well known, they have inherent drawbacks. Firstly, they are not sensitive to vertical vibrations though they can respond well to horizontal vibrations. Secondly, they are capable of being actuated when they are tilted by other effects. Thirdly, they do not respond to vibratory acceleration. This means that they cannot ensure reliable actuation by vibrations of energy above a predetermined magnitude and their response time fluctuates. Fourthly, their output is not large, and efforts to increase their output leads to increased size and weight of the whole construction due to the increased weight of the ball or bar and also to a complicated construction.